No matter whether you change an original model/map/other piece of work, it is technically using it. And if you publish it without getting permission, then it is technically stealing another modder's work, which you have used as a basis for your own changes. If you want to not steal another modder's work, you have to start from scratch (or, obviously, receive permission to use the modder's work and change it). It's quite stupid to think that if you take f.e. the model and textures of the above-said sword of Aragorn from, let's say, Ak Ela, do some minor or bigger changes to it and then publish the mod with it without having received permission from the Ak Ela team to use that sword (be it as they've made it or just as a basis for your changes), it wouldn't be stealing. Because it would. I can tell you that in many cases when a modder gives his work to another mod, his work is changed (f.e. in BGTW we've given our Bulgar unit-roster to many mods, but many of them have added some changes to fit their style better, sometimes even at our own requests), but you still need permission from that modder, since you're using his work as your basis. Now, in the case of Anduril, if you make the models and textures from scratch, all on your own, then you're obviously perfectly fine with using it and giving permissions to others to use and/or change it.
This also comes to explain your misconception between inspiration and stealing - when making the maps, scene props, equipments and all other kinds of things, based on some already existing book or movie (like Tolkien's works or the Star Wars examples you mentioned), then you obviously have to draw inspiration from them, since you'd have to make it as close to the original as you can. However, you still have to make it from scratch. If, let's say that you manage to hack into Peter Jackson's or George Lucas' computers, find there some files which contain the afore mentioned maps, scene props, equipments etc., take them and use them in your work, without having the permission of PJ or GL (or whoever made those respective things), then it's stealing, because you're not just inspired, but you're actually using someone else's work without permission.